In recent years, a display apparatus with a front panel such as a touch panel or a reinforced glass mounted on a surface of a display module including a display panel has been more widespread. The touch panel is used as an input device for the display apparatus. The reinforced glass is used for protecting the display surface. The reinforced glass may be decorative-printed from the perspective of improved design of the display apparatus.
A method of placing the front panel in the display module includes a method of bonding the display panel with the front panel in the display module at a peripheral part with the use of an adhesive or the like. An air space is present between the display panel and the front panel. This bonding method is called air gap bonding for convenience. Here, reflection of light occurs at the interfaces on both sides of the air space, since the refractive index of the front panel and display panel is greatly different from the refractive index of the air space. Thus, the air gap bonding has a problem of significantly reducing the visibility of a displayed image. This phenomenon of the significant deterioration in visibility is particularly notable under the external light environment.
In order to alleviate the deterioration in visibility, a method of bonding the display panel with the front panel by filling the air space with optical clear resin (hereinafter referred to as OCR) having a refractive index close to that of the front panel is employed. It is possible to suppress the reflection at the interface and to improve the visibility of the displayed image. This technique is called optical bonding or direct bonding. The technique has widely been employed also because of a mechanical effect such as enhancement in the impact resistance and anti-scattering at breakage for the front panel.
The upper and lower substrates (hereinafter also collectively referred to as an upper-lower substrate) constituting the display panel is, however, susceptible to external stress in terms of their structures. The local stress applied to the upper-lower substrate induces a display defect such as unevenness or color change. Because a stress is applied to the display module through the front panel in the display apparatus described above, designing a new product and a new manufacturing process are desired.
The structure and manufacturing process of the display apparatus to which the front panel is mounted through the OCR as described earlier are roughly grouped into two.
Japanese Patent Application Laid-Open No. 2010-066711 (hereinafter referred to as Patent Document 1) discloses a housing-out technique in which the front panel is bonded to the surface of the upper-lower substrate constituting the display panel and is thereafter held between a casing and a housing. Japanese Patent Application Laid-Open No. 2009-175701 (hereinafter referred to as Patent Document 2) discloses a housing-in technique in which a front panel is bonded to the surface of a display module including upper and lower substrates. Japanese Patent Application Laid-Open No. 2013-088455 (hereinafter referred to as Patent Document 3) discloses the physical property of a resin material used in the housing-out technique.
The housing-out technique in the former case requires a dedicated design on the assumption that the front panel is to be mounted, which makes it difficult to share components and thus increases the cost of product development. It is however easy to take design measures for a product. Therefore, the housing-out technique may be regarded as a process directed at products of a large lot production, which can sufficiently recover the cost of development. In the case of the housing-out technique, the front face of the front panel and the rear face of the upper-lower substrate are planarly held and uniformly pressurized from both sides. Even in the case where the OCR leaks out over the resin member which seals the OCR, the leaking OCR may be cured later by ultra violet (UV) curing resin used for the OCR. Thus, the manufacturing thereof is relatively easy.
While the housing-in technique described in the latter case is, contrary to the former case, easy to share components or to recover initial expenses, it is difficult to take design measures. Thus, it is often required to solve problems in the manufacturing process. Therefore, the housing-in technique may be regarded as a process directed to a small lot for which the cost of manufacturing is desirably suppressed to the minimum. Since the rear face of the upper-lower substrate is not supported by any part other than the periphery of the casing, it cannot be planarly or uniformly pressurized. If the OCR leaks out over the resin member, the leaking OCR may not be UV-cured. The OCR thus contaminates the inside of the display module. A method of solving such problems has been sought for.
Meanwhile, as for the visibility, the air gap bonding which is inferior to the optical bonding has such an advantage that manufacturing may be carried out at a relatively low cost without the use of OCR. Also in the case of air gap bonding, a problem arises in lowering of the display quality due to intrusion of abnormal substances to the air gap between the display module and the front panel.
It is therefore required to enhance the display quality as in the optical bonding, not only simply bonding the front panel with the display module.